Method of and apparatus for producing a repetitive seismic impulse

ABSTRACT

The improvement in the method and apparatus of producing a seismic source in water by a combustive explosion by reducing the oscillation of the explosive exhaust gas bubble in the water thereby reducing extraneous seismic impulses. Injecting an air/fuel mixture of less than the stoichiometric quantity into the chamber to provide an excess of air, and after the mixture has been exploded and released into the water, injecting additional fuel into the free air in the exhaust gases which provides additional combustion to increase the pressure in the exhaust gases to dampen the oscillation of the pressure in the gases. Injecting additional fuel into the hot exhaust gases and excess air for maintaining the exhaust gas bubble pressure at or above the water pressure surrounding the exhaust gases.

July 3, 1973 l I l METHOD OF AND APPARATUS FOR PRODUCING A REPETITIVESEISMIC IMPULSE Inventor: Charles D. Wood, [1], San Antonio,

Tex.

Assignee: Southwest Research Institute, San

Antonio, Tex.

Filed: Dec. 22, 1971 Appl. No.: 210,764

[52] US. Cl 340/12 SD, 181/.5 NC

[5|] Int. Cl. l-l04b 13/00 [58] Field of Search 340/12 SD, 12; 181/.5NC, .5 KC

[ 56] References Cited UNITED STATES PATENTS 3,397,755 8/l968 Loperl8l/.5 NC 3,506,085 4/l970 Loper l8l/.5 NC

Primary ExaminerBenjamin A. Borchelt Assistant ExaminerH. J. TudorAttorney-James F. Weiler, William A. Stout and Dudley R. Dobie, Jr. etal.

[57] ABSTRACT The improvement in the method and apparatus of producing aseismic source in water by a combustive explosion by reducing theoscillation of the explosive exhaust gas bubble in the water therebyreducing extraneous seismic impulses. Injecting an air/fuel mixture ofless than the stoichiometric quantity into the chamber to provide anexcess of air, and after the mixture has been exploded and released intothe water, injecting additional fuel into the free air in the exhaustgases which provides additional combustion to increase the pressure inthe exhaust gases to dampen the oscillation of the pressure in thegases. Injecting additional fuel into the hot exhaust gases and excessair for maintaining the exhaust gas bubble pressure at or above thewater pressure surrounding the exhaust gases.

6 Claims, 6 Drawing Figures BACKGROUND OF THE INVENTION DESCRIPTION OFTHE PREFERRED EMBODIMENT Referring now to FIG. 1, the apparatus of theGenerally,it is old to provideapneumatic oracom- 5 present invention isprovided to produce a seismic bustive fuel/air explosion for providing arepetitive seismic impulse in water. However, as noted in U.S. Pat. No.3,397,755, secondary pressure pulses normally occur upon the release ofpressurized air into water caused by the oscillation in size of the airbubble. Similarly, underwater seismic sources that produce seismicimpulses by the release of a combustive air/fuel explosion also form anoscillating exhaust gas bubble which expands and contracts to causeundesirable periodic energy impulses in the water after the initialimpulse. These extraneous impulses confuse the information obtained fromthe selected pulse being measured. The present invention is directed toreducing or eliminating the oscillation of the hot exhaust gas bubblefrom the combustion chamber to thereby eliminate or reduce the undesiredseismic impulses caused by the oscillation.

SUMMARY The present invention is directed to a method and apparatus forreducing exhaust gas oscillations in a seismic impulse produced in waterby a combustive fuel/air explosion by injecting a hydrocarbon fuel andair into the combustion chamber in a mixture less than stoichiometricthereby providing additional or free air after exploding the mixture,and releasing the exhaust gases into the water, injecting additionalfuel into the free air in the exhaust gases to cause further explosionsto increase the pressure within the exhaust gases for damping theoscillations of the gases.

The present invention is further directed to injecting additional fuelinto the free air in the released exhaust gases in a rate and amount tomaintain the gas pressure in the exhaust gases at least as great as thewater pressure surrounding the exhaust gases.

A still further object of the present invention is the provision ofexhaust ducts in communication with the exhaust valve in the combustionchamber of a combustive type seismic source into which additional fuelcan be injected into the exhaust gases for eliminating or reducing theundesired energy pulses transmitted to the water by the exhaust gases.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational view, incross section, and partly schematic, illustrating the apparatus of thepresent invention,

FIG. 2 is a fragmentary elevational view, showing the exhaust dischargeend of the combustion chamber of FIG. 1,

FIG. 3 is a cross sectional view taken along the line 3-3 of FIG. 2,

FIG. 4 is an enlarged fragmentary cross sectional view illustrating theinjection of additional fuel into the exhaust gases of the apparatus ofFIG. 1,

FIG. 5 is a graph illustrating the oscillation of the bubble size ofexhaust gases from the apparatus of FIG. 1, and

FIG. 6 is a graph illustrating variation of pressure versus time as theexhaust gases bubble oscillates as shown in FIG. 5.

shock in water for the purpose of seismic prospecting and generallyincludes a combustion chamber 12, air supply means 14 for injectingcompressed air into the chamber 12, and fuel injection means 18 forinjecting a hydrocarbon fuel, preferably diesel fuel. The air supplymeans 14 for introducing compressed air may include a check valve 16 andthe pressure of the air introduced may be approximately 300 psi, but canbe in the range from about 200 to 600 psi. When the combustion chamber12 is charged with air, the fuel injecting means 18 such as a solenoidvalve opens to allow a finely atomized spray of diesel fuel to enterinto the chamber I2. A supply of pressurized diesel fuel is maintainedat the inlet of the solenoid operated fuel valve 18 so that the quantityof injected fuel can be controlled by the length of time the fueldelivery valve 18 remains open.

If the combustion chamber 12 is hot from previous firings and if theentering air temperature is sufficiently high, the injected diesel fuelwill ignite spontaneously with a short delay between fuel entry andignition. However, if the chamber 12 is cold, additional ignition meanssuch as a heater element 20 may be provided inserted through the wall ofthe combustion chamber 12 to initiate combustion. Under thesecircumstances, the fuel is ignited and burns, raising the temperatureand pressure in the combustion chamber, for example, to approximately5,000 F. and L500 psi, respectively.

An exhaust valve generally indicated by the numeral 22 is located in thecombustion chamber 12 to release the hot explosive exhaust gases intothe water to provide a seismic impulse. The exhaust valve 22 may includea poppet valve 24 attached to a piston 26 and includes a spring 28acting to normally close the poppet valve 24. Pressure in the combustionchamber 12 is transmitted to chamber 30 and acts on the back side of thepoppet valve 24 and on side 32 of the piston 26. Since piston 26 isgreater in cross-sectional area than the back side of the poppet valve24, the poppet valve 24 remains closed. However, a control line 34 incommunication between the combustion chamber 12 and the back side 36 ofpiston 26 is provided and the flow therein is controlled by solenoidvalve 38. When the valve 38 is in position 40 as shown, the pressureacting on the side 36 of piston 26 is equal to that of the surroundingsand the poppet valve 24 is closed. Check valve prevents the entry ofwater into the volume above the piston 32. However, when the solenoidvalve 38 is placed in position 42 in the line 34, pressure from thecombustion chamber 12 is transmitted to side 36 of the piston 26balancing the forces on the piston 26 allowing the poppet valve 24 toopen, and release the explosive gases from the chamber 12. The exhaustgases cause a pressure wave in the surrounding water which is useful forseismic prospecting. The apparatus I0 is capable of continuous,repetitive explosions at a rate dependent upon the quantity ofcompressed air available up to a minimum of about two explosions persecond.

Underwater seismic sources that produce energy by the release of a gasinto the water have been found to produce an energy pulse of largeamplitude and for this reason are widely used for seismic exploration.However, this type of source has a significant disadvantage. When theexhaust gases are released into the water a gas bubble forms and growsin size while the gases are being fed into it. As the bubble grows, theinertia of the moving mass of water around the bubble causes the gasbubble to continue growing so that the gas pressure in the bubble mayfall well below the surrounding pressure of the water. Then, because ofthe lower pressure within the bubble, the bubble begins to contract.Again due to the inertia of the water surrounding the gas bubble, thecontraction continues past the point at which the gas pressure equalsthe water pressure and therefore the pressure in the gas bubble israised to a valve well above the water pressure. The diameter of thebubble at this stage is correspondingly small. A typical oscillation isseen in FIG. 5 of a graph showing changes in diameter of an exhaust gasbubble. Thus the exhaust gases from the combustion chamber 12 form a gasbubble 99 at the time the exhaust gases are released through the exhaustvalve 22. However, the inertia of the moving water away from the bubbleallows the bubble to increase in size at positions 50, 52 and 54, atwhich time the pressure in the bubble has fallen below that of thesurrounding pressure of the water. The bubble is contracting due to thegreater water pressure at positions 56, 58 and 59. Thus at position 59,the gas pressure in the bubble is greater than the pressure of the waterand the bubble then again begins to expand at positions 60, 62 and 64.These oscillations of the size of the bubble cause periodic impulses inthe water after the initial pulse 65, as best seen in the graph 66 ofFIG. 6. The extraneous pulses resulting from the oscillations of thebubble, such as pulse 68, are undesirable and confuse the informationobtained from the reflected pulses from the strata at the bottom of thewater.

The present invention is directed to damping the oscillation of theexhaust gas bubbles and thereby reduce or eliminate the undesiredperiodic energy impulses produced by the exhaust gases after the initialpulse. Thus, the fuel/air mixture supplied to the combustion chamber 12is provided in less than the stoichiometric quantity to provide a supplyof air or oxygen in the exhaust gases after combustion. This may beobtained by charging the gomb stion chamber with air in the usualmannerj fiirtre'dugl the amount of fuel injected so that aftercombustion ,iswpmpleted, there remains in the chamber a certainamouritofair containing oxygen which is not burned. The exhaust gases,now also containing free or unburned oxygen, are released through theexhaust valve 22. Adjacent the exhaust valve 22 one or more small fuelinjectors 70 are proyided to spray fuel into the hot exhaust gasesbeingrleased from the combustion chamber 12 at a controlled rate. Whileany suitable means may be used to operate the injector valves 70; by wayof example only, solenoid valves 72 may be provided connected to andcontrolling the injector valves 70 and in time are suitably controlledand programmed. Thus, for example, the injectors 70 may be initiated andcontrolled by an electrical signal automatically programming thesolenoid valves 72 using the opening of the exhaust valve 22 as areference. The fuel from the injectors 70 burns by combining with theoxygen or free air in the hot exhaust gases thereby increasing thepressure within the bubble during its expansion to build up pressuretherein to prevent contraction of the bubble thereby damping theoscillation of the exhaust gas bubble. The rate and amount of fuelinjected is controlled by the surrounding water pressure and theinjector characteristics, and is preadjusted to produce the maximumdamping effect on the volume of gas released by the seismic energysource at the normal operating depth of the source. In any event, thefuel injection into exhaust gases is such that the oscillation of thebubble of exhaust gases is prevented or reduced to eliminate or reducethe energy pulses transmitted to the water by the oscillating bubble bymaintaining the exhaust bubble pressure at or above the pressure of thewater surrounding the gas exhaust bubble.

Preferably, an exhaust duct 74 is in communication between the exhaustvalve 22 and the water and includes one or more outlets 76, and theoutput from the fuel injectors is directed to spray fuel into the duct70 thereby insuring that the fuel combines with the exhaust gases andfree air at a location where the pressures and temperatures areconducive to ignition of the added fuel.

in use, the apparatus 12 is towed through the water by a suitable vessel(not shown). A conventional air compressor, fuel pump and suitablecontrols (not shown) are on the towing vessel and the combustion chamberis connected to the vessel by a towing cable. At the desired location,air is introduced into the combustion chamber 12 through the air supplymeans 14 and fuel is sprayed therein such as through fuel injectionvalve 18 and ignited, if necessary, by ignition means 20 to cause anexplosion therein. As previously mentioned, the fuel/air mixture is lessthan the stoichiometric quantity in order to insure that a certainamount of oxygen in the free air remains after combustion of the fuel iscompleted. The exhaust valve 22 is actuated by the solenoid valve 38 toopen the poppet valve 24 and exhaust the hot explosive gases into thewater through the duct 74 and outlets 70 to provide a seismic impulse.Additional fuel is injected into the free air carried by the exhaustgases to provide additional combustion thereby maintaining the gaspressure in the exhaust gas bubble at least as great as the pressure ofthe water surrounding the gas bubble.

lt is believed that the method of the present invention is apparent fromthe foregoing description of the apparatus. However, the methodcomprehends reducing the gas oscillations in a seismic impulse producedin water by a combustive explosion by injecting air and diesel fuelunder pressure into a combustion chamber in a fuel/air mixture less thanstoichiometric, exploding the mixture, releasing the explosive gases andfree air through an exhaust valve and into the water to provide c aseismic impulse, and injecting additional diesel fuel into the explosivegases and free air for maintaining the released gas pressure in the gasbubble at least as great as the pressure of the water surrounding thegas.

The present invention, therefore, is well adapted to carry out theobjects-and attain the ends and advantages mentioned as well as othersinherent therein.

What is claimed is:

l. A method of producing, for the purposes of sei smic surveying, apressure wave in water by a combustive explosion having reduced gasoscillations comprising,

injecting a hydrocarbon fuel and air into a combustion chamber in afuel/air mixture less than stoichiometric thereby providing excess air,

exploding the mixture,

releasing the explosive gases into the water through an exhaust valve,

injecting additional fuel into the released explosive gases includingexcess air thereby increasing the pressure within the gases for dampingthe oscillations of the gases.

2. The method of claim 1 wherein,

the additional fuel is injected with reference to the actuation of theexhaust valve.

3. The method of claim 1 wherein,

the rate and amount of additional fuel injected is to maintain theexhaust gas pressure at least as great as the water pressure.

4. A method of reducing gas oscillations in a seismic impulse producedin water by a combustive explosion comprising,

injecting air and diesel fuel under pressure into a combustion chamberin a fuel/air mixture less than stoichiometric thereby providing excessfree air, exploding the mixture,

releasing the explosive gases and free air from the chamber through anexhaust valve and into the water to provide a seismic impulse, and

injecting additional diesel fuel into the explosive gases and free airfor maintaining the released gas pressure at least as great as thepressure of the water surrounding the gas.

5. An apparatus for producing seismic pulses in water 5 comprising,

a combustion chamber for contacting the water and having an exhaustvalve,

first means for injecting a hydrocarbon fuel and air under pressure intothe combustion chamber in a fuel/air mixture less than stoichiometric,

means for exploding the mixture,

means for opening the exhaust valve and releasing the explosive gasesinto the water,

second fuel injection means positioned adjacent the exhaust valve forinjecting additional fuel into the explosive gases, and

control means for controlling the second injection means for maintainingthe pressure in the explosive released gas at least as great as thepressure of the water surrounding the released gases.

6. The apparatus of claim 5 including,

an exhaust duct in communication between the exhaust valve and the waterand into which the fuel from the second fuel injection means isdirected.

i I It i t

1. A method of producing, for the purposes of seismic surveying, apressure wave in water by a combustive explosion having reduced gasoscillations comprising, injecting a hydrocarbon fuel and air into acombustion chamber in a fuel/air mixture less than stoichiometricthereby providing excess air, exploding the mixture, releasing theexplosive gases into the water through an exhaust valve, injectingadditional fuel into the released explosive gases including excess airthereby increasing the pressure within the gases for damping theoscillations of the gases.
 2. The method of claim 1 wherein, theadditional fuel is injected with reference to the actuation of theexhaust valve.
 3. The method of claim 1 wherein, the rate and amount ofadditional fuel injected is to maintain the exhaust gas pressure atleast as great as the water pressure.
 4. A method of reducing gasoscillations in a seismic impulse produced in water by a combustiveexplosion comprising, injecting air and diesel fuel under pressure intoa combustion chamber in a fuel/air mixture less than stoichiometricthereby providing excess free air, exploding the mixture, releasing theexplosive gases and free air from the chamber through an exhaust valveand into the water to provide a seismic impulse, and injectingadditional diesel fuel into the explosive gases and free air formaintaining the released gas pressure at least as great as the pressureof the water surrounding the gas.
 5. An apparatus for producing seismicpulses in water comprising, a combustion chamBer for contacting thewater and having an exhaust valve, first means for injecting ahydrocarbon fuel and air under pressure into the combustion chamber in afuel/air mixture less than stoichiometric, means for exploding themixture, means for opening the exhaust valve and releasing the explosivegases into the water, second fuel injection means positioned adjacentthe exhaust valve for injecting additional fuel into the explosivegases, and control means for controlling the second injection means formaintaining the pressure in the explosive released gas at least as greatas the pressure of the water surrounding the released gases.
 6. Theapparatus of claim 5 including, an exhaust duct in communication betweenthe exhaust valve and the water and into which the fuel from the secondfuel injection means is directed.